Cassava is a major food crop, providing the dietary needs of over a half a billion people in the developing world. It is commonly cultivated in regions of drought, where according to Cruz et al. (2016), "even under adverse soil and climatic conditions, cassava can produce a satisfactory root yield, while other annual crops barely survive (El-Sharkawy and Cock, 1987)." Despite its production prominence and importance to food security, however, Cruz et al. write in the introduction of their paper that "little is known about how cassavas respond to predicted atmospheric global changes." Thus, it was the objective of this team of five researchers to conduct a study "designed to help understand the interaction between elevated CO2 levels and water deficit on growth, physiology and dry mass accumulation in cassava." Their work was conducted at the USDA-ARS Crops Research Laboratory in Fort Collins, Colorado, where they grew 2- to 3-month-old cassava plantlets in a climate-controlled greenhouse for 100 days at two CO2 concentrations (390 or 750 ppm for 12 hours per day) and two water treatments (well-watered and water-stressed).

This effort revealed that (1) "water deficits led to reductions in the Leaf Elongation Rate of plants grown at ambient as well as CO2-enriched concentrations." However, they report that (2) "plants grown at 750 ppm of CO2 maintained leaf growth two days longer than plants grown at 390 ppm." Then, (3) "three days after withholding water, photosynthesis and stomatal conductance were reduced in plants grown under ambient CO2, while [4] in plants under an elevated CO2 concentration, these physiological functions remained similar to that of control plants grown under good water availability."

Continuing, the five researchers report that (5) "five days after withholding water plants grown with 750 ppm continued to have enhanced gas exchange compared with plants grown under 390 ppm." Also, (6) "under drought stress, the instantaneous transpiration efficiency was always greatest for plants grown under elevated CO2." And, last of all, they write that (7) "the positive response of elevated CO2 levels on total dry mass was 61% in the water-stressed plants and only 20% for the plants grown under good water availability" (see figure below). Thus, as the CO2 concentration rises in the future, its biomass-enhancing and drought-alleviating effects on cassava should be welcomed by those who depend on this important food crop.

Figure 1. Effect of CO2 on the total dry mass of cassava under well-watered and water-stressed conditions. Adapted from Cruz et al. (2016).